vect: Refit lane-reducing to be normal operation

Vector stmts number of an operation is calculated based on output vectype.
This is over-estimated for lane-reducing operation, which would cause vector
def/use mismatched when we want to support loop reduction mixed with lane-
reducing and normal operations. One solution is to refit lane-reducing
to make it behave like a normal one, by adding new pass-through copies to
fix possible def/use gap. And resultant superfluous statements could be
optimized away after vectorization.  For example:

  int sum = 1;
  for (i)
    {
      sum += d0[i] * d1[i];      // dot-prod <vector(16) char>
    }

  The vector size is 128-bit，vectorization factor is 16.  Reduction
  statements would be transformed as:

  vector<4> int sum_v0 = { 0, 0, 0, 1 };
  vector<4> int sum_v1 = { 0, 0, 0, 0 };
  vector<4> int sum_v2 = { 0, 0, 0, 0 };
  vector<4> int sum_v3 = { 0, 0, 0, 0 };

  for (i / 16)
    {
      sum_v0 = DOT_PROD (d0_v0[i: 0 ~ 15], d1_v0[i: 0 ~ 15], sum_v0);
      sum_v1 = sum_v1;  // copy
      sum_v2 = sum_v2;  // copy
      sum_v3 = sum_v3;  // copy
    }

  sum_v = sum_v0 + sum_v1 + sum_v2 + sum_v3;   // = sum_v0

2024-07-02 Feng Xue <fxue@os.amperecomputing.com>

gcc/
	* tree-vect-loop.cc (vect_reduction_update_partial_vector_usage):
	Calculate effective vector stmts number with generic
	vect_get_num_copies.
	(vect_transform_reduction): Insert copies for lane-reducing so as to
	fix over-estimated vector stmts number.
	(vect_transform_cycle_phi): Calculate vector PHI number only based on
	output vectype.
	* tree-vect-slp.cc (vect_slp_analyze_node_operations_1): Remove
	adjustment on vector stmts number specific to slp reduction.

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